How the genetic code was cracked

possible 3-letter codes (image credit)
The structure of DNA, solved in 1953, set off a race to crack the genetic code. How do sequences of 4 nucleotides code for sequences of 20 amino acids? This coding problem lies at the heart of molecular biology. Physicist George Gamow of Big Bang fame contributed the first guess: Spaces between neighboring nucleotides might fit individual amino acids, directly templating protein assembly on the DNA. In Gamow's solution, each nucleotide must contribute to defining two amino acids–an overlapping code. The numerology looked good (there were exactly 20 possible combinations), but Gamow's solution turned out to be dead wrong: In 1957, Sydney Brenner devised a clever test that disproved this and all overlapping triplet codes. The true code was soon cracked based on beautiful frameshift experiments by Crick and colleagues (proving a triplet code), and by analysis of proteins synthesized from artificial RNAs (solving each codon).
Supplements: Gamow's guess, Brenner disproves Gamow and all overlapping triplet codes, the decisive artificial RNA experiments

How do growing nerve cells find their targets?

Rita Live-Montalcini (image credit)
In 1949, Rita Levi-Montalcini noticed something unexpected. Her colleague Elmer Bueker had found that nerves would invade tumors implanted into chick embryos, but what attracted the nerves to tumors? Indeed how did nerves ever find their normal targets? What Levi-Montalcini had noticed: the nerves were also invading the tissues near the tumors, even invading blood vessels downstream of tumors — suggesting that the tumors might have been releasing a diffusible nerve growth factor (NGF), a postulated substance that could guide either nerve differentiation, growth or survival. Levi-Montalcini proved the existence of NGF by culturing just tumors and ganglia in the same dish, finding that the nerves from the ganglia would connect to tumors even in vitro. Later, she purified NGF. NGF told us that the way nerves find their targets is unexpectedly adaptive: nerves grow just about everywhere, and they die off where they fail to find targets. 
A short review: Aloe, L. (2004) Rita Levi-Montalcini: the discovery of nerve growth factor and modern neurobiology. Trends Cell Biol 14:395-9. 

Some amazing historical background: An excerpt about her pre-NGF work, which she did in a makeshift home lab that she set up hiding out from Nazis and fascists during WWII, from her autobiography, In Praise of Imperfection. Open the excerpt in Acrobat and you'll see some helpful notes in red.

How the widowbird got its absurdly long tail

Wikimedia Commons
One glaring difficulty for early evolutionary biologists was the evolution of exaggerated male secondary sexual traits that would hinder the survival of males. Darwin was the first to suggest that sexual selection favoring an exaggerated trait could 'override' natural selection opposing the same trait, as long as the reproductive benefits outweighed the survival costs associated with it. During the modern synthesis, Ronald Fisher proposed a mechanism of ornament evolution wherein female preferences for an initially advantageous male trait drove that trait to an extreme where it no longer conferred a survival advantage. But little empirical evidence existed even into the early 1980s.  Andersson's work on long-tailed widowbirds provided the first experimental evidence of female preference for an extreme male ornament. This beautifully designed manipulation study was able to control for the confounding influence of variation in male territory quality, the experimental manipulation, and male behavior.
And a later paper summarizing subsequent work on widowbirds